The present invention relates to antennas and more particularly to an improved planar monopole antenna capable of operating at two different frequency ranges.
Portion of a conventional antenna having parallel rods (i.e., so called Lecher wires) mounted on a TV is shown FIG. 1. Opposite current flows as indicated by arrows will be induced on two parallel metal (e.g., copper) radiating rods 14 of the antenna when they are close enough. Also, respective electromagnetic fields are generated around the radiating rods 14 by the induced current. But the electromagnetic fields will be cancelled each other due to opposite directions, resulting in a prohibition of radiation. For enabling the antenna to effectively radiate electromagnetic waves in a narrow space, the open ends of the radiating rods 14 are bent about 90 degrees in opposite directions to form signal feed lines 242 as shown in FIG. 2. As a result, current flows on the signal feed lines 242 are in the same direction as indicated by arrows. This antenna is so called dipole antenna. The dipole antenna comprises two parallel rods as feed lines 24 in a structure of balance transmission line. Portions of the feed lines 24 as implemented in the structure of balance transmission line are bent to form the above signal feed lines 242 which are extended the same lengths. A length of each signal feed line 242 is about one-quarter wavelength at a resonant frequency (e.g., xcex/4 where xcex is wavelength at the resonant frequency). In other words, a total length of the signal feed lines 242 is about one half wavelength at the resonant frequency (e.g., xcex/2). As such, the signal feed lines 242, each having about one-quarter wavelength, are used by the antenna as radiating elements. Such antenna is also called half wave dipole antenna which is typically operated at a single frequency.
For making the conventional antenna more compact, a technique of manufacturing the antenna on a printed circuit board is adopted by some manufacturers in the art as shown in FIGS. 3 and 4. This kind of patch antenna comprises a dielectric substrate 37, a patch line 34 printed on the top of the dielectric substrate 37, the patch line 34 having one end formed as a signal feed point 341, a ground metal plate 38 printed on the bottom of the dielectric substrate 37 opposite to the patch line 34, and an inverted L-shaped radiating element 342 formed at the other end of the patch line 34, the inverted L-shaped radiating element 342 being extended in a direction perpendicular to the patch line 34 above and beyond the ground metal plate 38, forming a so-called monopole antenna. The monopole antenna takes advantage of image theory employed by the ground metal plate 38 to map the patch line 34 and the inverted L-shaped radiating elements 342 of this structure of unbalanced transmission line. As an end, an antenna having radiating elements equivalent to the above dipole antenna is formed. The antenna is also typically operated at a single frequency.
There has been a significant growth in wireless local Area network (WLAN) due to an increasing demand of mobile communication products in recent years in which IEEE 802.11 WLAN protocol is the most important one among a variety of WLAN standards. The IEEE 802.11 WLAN protocol was established in 1997. The IEEE 802.11 WLAN protocol not only provides many novel functions for WLAN based communication but also proposes a solution for communicating between mobile communication products made by different manufacturers. There is no doubt that the use of the IEEE 802.11 WLAN protocol is a milestone in the development of WLAN. The IEEE 802.11 WLAN protocol was further modified for being adapted to serve as a standard of both IEEE/ANSI and ISO/IEC in August 2000. The modifications comprise IEEE 802.11a WLAN protocol and IEEE 802.11b WLAN protocol. In an expanded standard physical layer, the operating frequencies have to be set at 5 GHz and 2.4 GHz. As such, the well-known L-shaped antenna cannot satisfy the requirement of enabling a mobile communication product to use both IEEE 802.11a and IEEE 802.11b WLAN protocols at the same time. Instead, several antennas have to be mounted in the product for complying with the requirement of frequency band. However, such can increase a manufacturing cost, complicate an installation procedure, and consume precious space for mounting the antennas. As a result, the size of the product cannot be reduced, thereby contradicting the compactness trend.
A primary object of the present invention is to provide a planar monopole antenna capable of operating at two different frequency ranges for fulfilling the need of multi-frequency operation which is unobtainable by a conventional monopole antenna only operated at a single frequency.
One object of the present invention is to provide a planar monopole antenna operable at two different frequency ranges comprising a dielectric substrate; a patch line printed on a top of the dielectric substrate, the patch line having one end formed as a signal feed point; a ground metal plate printed on a bottom of the dielectric substrate; a first radiating element operated at a low frequency extended from the other end of the patch line beyond the ground metal plate and being perpendicular to the patch line in either direction, the first radiating element operated at a low frequency being further extended a predetermined distance in a direction parallel to the patch line toward and spaced apart from the ground metal plate in which a length of the first radiating element operated at a low frequency extended from the patch line beyond the ground metal plate is about one-quarter wavelength at a low operating frequency of the frequency ranges; and a second radiating element operated at a high frequency projected from a side of the patch line beyond the ground metal plate; the second radiating element being spaced apart from the first radiating element operated at a low frequency. By utilizing the antenna, the radiating elements can receive signals of dual frequency.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings.